In the hot-dip galvanising process, steel strip is thoroughly cleaned by acid pickling, alkaline cleaning or other preparatory treatment and is then generally passed through an annealing furnace and kept under the protection of a reducing furnace atmosphere until it passes into a bath of coating metal held in a coating pot. The coating metal is usually maintained molten in the coating pot by the use of heating inductors. The strip may pass through an elongate furnace exit chute or snout which dips into the bath. Within the bath the strip passes around one or more sink rolls and is taken upwardly out of the bath. After leaving the coating bath the strip may pass through a gas knife or gas wiping station in which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.
In a normal galvanising process the coating metal may be zinc or zinc with about 0.2% of aluminium by weight. This produces a standard galvanised steel which has moderate corrosion resistance and can be produced at moderate cost. Superior coatings can be obtained with zinc and aluminium alloys having a much higher aluminium content, for example in the range 25-70%.
There is presently a need to provide a hot-dip coating plant which can be operated in alternative modes either to produce highly corrosion resistant strip coated with a zinc-aluminium alloy of high aluminium content or alternatively to produce standard galvanised strip with a predominantly zinc coating. In particular, there are many existing plants around the world producing the high corrosion resistant strip coated with zinc-aluminium alloy in locations where there is still a demand for the standard galvanized strip and there is accordingly a need to be able to convert existing plant for production of both kinds of strip.
It is not commercially or technically attractive to use the same coating pot to hold both kinds of coating metal. This would require the pot to be totally pumped out or emptied on changing from one form of coating to the other, which will inevitably result in damage to the coating metal heating inductors and refractories in the pot. It is also extremely time consuming and generates metals which need to be reclaimed. Moreover, it is very difficult to produce a bath of almost pure zinc in a pot which has previously contained a metal with a high proportion of aluminium.
It is known to provide two fixed coating pots side by side and make parts of the strip feed lines and furnace moveable to line up alternatively with one pot or the other. However, this is extremely expensive and involves difficult engineering of the moveable parts. It is also known to move and replace the coating pot with a similar substitute pot but this is also extremely expensive and time consuming. The present invention provides an alternative solution by which a main coating pot can be used for coating strip with a first coating metal and a second, shallower coating pot can be positioned within the upper part of the main coating pot to enable coating with a second coating metal.